Enhancement: Add key hinting

Added a new option `--key-hint` and corresponding environment variable
`RESTIC_KEY_HINT`.  The key hint is a key ID to try decrypting first, before
other keys in the repository.

This change will benefit repositories with many keys; if the correct key hint
is supplied then restic only needs to check one key.  If the key hint is
incorrect (the key does not exist, or the password is incorrect) then restic
will check all keys, as usual.

https://github.com/restic/restic/issues/2097
